Method and medical device designed for implementing this method

ABSTRACT

A method is for restoring a signal of a defective channel of a beam detector, which has a multitude of channels and by means of which projections are recorded from different directions of projection. The channels of the beam detector each have a detector element with channel electronics connected downstream thereform. The signal of the defective channel is restored while using neighboring signals of an M-neighborhood of the same projection and from adjacent signals of an M-neighborhood of additional projections.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/DE02/03056 which has an Internationalfiling date of Aug. 21, 2002, which designated the United States ofAmerica and which claims priority on German Patent Application number DE101 43 045.0 filed Sep. 3, 2001, the entire contents of which is herebyincorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to a method for restoring a signal of adefective channel of a radiation detector that has a multiplicity ofchannels and/or by which projections from different projectiondirections are recorded, and/or whose channels in each case have adetector element with a downstream channel electronic unit. Theinvention also generally relates to a medical device, in particular a CT(computer tomography) device, designed for implementing this method.

BACKGROUND OF THE INVENTION

In medical diagnostics, for example, computer tomography serves thepurpose of producing non-overlapping tomographs. In the case of x-raycomputer tomography, these tomographs are calculated with the aid of acomputer from data that are recorded during the circular or spiralrevolution of an x-ray tube and a suitable detector about the patient(see Kalender, W. A.: Computertomographie. Grundlagen,Gerätetechnologie, Bildqualität, Anwendungen. [Computer tomography.Fundamentals, equipment technology, image quality, applications.]Publicis MCD Verlag, Munich, 2000). The aim is to obtain tomographs inthe shortest possible time.

Because of their excessively slow data collection rate, conventional CTdevices having a so-called single-row detector, that is to say adetector that has a single detector row or row-type arrangement ofdetector elements, are not capable of meeting the growing clinicaldemands (high-resolution recording of complete organs during a pause inbreath, large-volume angiographs, three-dimensional representations ofanatomical structures with isotropic and high resolution).

Although it would be possible to increase the data collection rate byreducing the time of revolution of the x-ray tube and detector,mechanical limits are soon encountered in so doing. In order,nevertheless, to permit a further increase in the data collection rate,CT devices have recently been developed which have a so-called multirowdetector, that is to say a detector that has several rows, for example 4rows, of detector elements (see Kalender loc. Cit.).

A data collection rate that is further increased by way of so-calledarea detectors, that is to say multirow detectors with a high number ofrows (for example 64 detector rows) is being aimed at, and is thesubject matter of the current development.

If a detector channel has defects or is entirely defective, itscorrupted or missing signal leads to inconsistencies in the total datavolume that have a disadvantageous effect on the quality of thereconstructed tomographs. For example, depending on the number ofchannels affected and on the type of defect, annular to linear artifactsappear that cover structures in the object being examined. In the mostfavorable case, they are merely perceived as disturbing by the viewer.But frequently, they influence the diagnosis disadvantageously or evenrender it entirely impossible.

FIG. 1 a illustrates a tomograph of the shoulder area of a human patientrecorded by a CT device with an entirely intact detector. If, forexample, 64 of 2688 channels of the detector are defective, a tomographin accordance with FIG. 1 b may result. In this case, it is severelyaffected by artifacts in the case in which the defective channels have aconstant signal level in each case independently of the number of x-rayquanta actually impinging on the corresponding detector element. For thesake of simplicity, the term defective channel will always be used belowindependently of whether a channel is entirely defective or onlyoperating in a defective way.

The cause of a defective channel can be both defects in the actualdetector element itself, and defects in the downstream electronic signalprocessing unit. Consequently, a defect can be eliminated by exchangingthe relevant detector element and/or the relevant part of the electronicsignal processing unit. Such an exchange is, however, time-consuming andcostly.

Methods have therefore been developed that render it possible to restorethe signals of defective channels in order thus to be able to dispensewith an exchange, or to be able to delay the latter at least until nointerruption of the operation of the CT device is necessary. Instead ofthe tomograph in accordance with FIG. 1 b, the application of acorrection method known from DE 199 21 763 A1 results in the tomographin accordance with FIG. 1 c which, although clearly having fewerartifacts, is far from being free of artifacts.

In the case of multirow detectors with N rows, the number of thechannels is N times the number of the channels present in the case of asingle-row detector. In the case of multirow detectors, in particular,however, in the case of area detectors, the probability of a defecttherefore rises by at least N times compared with a single-row detector.The economic use of multi-row and area detectors is therefore broughtinto question because of the high probability of frequently having toexchange detector elements or parts of the electronic signal processingunit.

SUMMARY OF THE INVENTION

It is an object of an embodiment of the invention to create thepreconditions for a practical and economic use of multirow and areadetectors by specifying a method, suitable for such detectors, forrestoring the signal of a defective channel, and a medical device forimplementing such a method.

Consequently, a signal to be restored is restored from the adjacentsignals of the respective current and at least one further projection.

Since recourse is also made to signals of at least one furtherprojection, a particularly realistic restoration is achieved with theconsequence that, as a rule, neither do disadvantageous influences arisefor the diagnosis, nor are artifacts perceived as disturbing orirritating by the viewer visible in the tomograph determined by usingthe restored signals. Further, this situation is achieved without theneed to exchange the defective channel or channels. Since the methodaccesses signals already present, it operates quickly enough to be ableto be used even in the case of high data collection rates or short cycletimes of the x-ray tube and detector.

As may be seen from FIG. 1 d, which shows a tomograph obtained with theaid of the method according to an embodiment of the invention from thesame signals as for FIG. 1 b, defects caused by defective or corruptedsignals actually no longer appear in the reconstructed tomograph. Thus,the method according to an embodiment of the invention really doessupply tomographs virtually free of artifacts.

In accordance with a variant of an embodiment of the invention, it isparticularly advantageous for the purpose of restoration to make use ofthe data of a so-called eight neighborhood of that projection in whichthe signal is to be reconstructed (termed current projection below), andof the data of the eight neighborhood of the projection directlypreceding the current projection in time (termed preceding projectionbelow).

It is also possible within the scope of an embodiment of the inventionto apply methods that make use of the data of extended neighborhoods,for example of twenty-four neighborhoods, and/or operate withsupplementary addition of data that are in front of the precedingprojection in time or after the current projection in time.

Moreover, an embodiment of the invention can also be applied in the caseof devices whose detector system does not rotate about a center ofrotation on a circular or spiral path relative to the object to beexamined, but moves along a trajectory of another type.

The terms “current projection”, “projection (directly) preceding (intime)” and “(temporally) adjacent projection” are used with regard tothe circumstance that the projections are recorded following one anotherin time so that a data stream corresponding to the projections isproduced. Consequently, adjacent projections are those projections whichrelate temporally, and therefore also spatially, to the currentprojection in such a way that the data contained in them are suitablefor restoring the signal of a defective channel with reference to thecurrent projection. As mentioned, adjacent projections can be after thecurrent projection in time, but also in front of the (directly)preceding projection.

Further advantages, features and details of the invention will becomeevident from the description of illustrated embodiments givenhereinbelow and the accompanying drawings, which are given by way ofillustration only and thus are not limitative of the present invention,wherein:

FIG. 1 shows tomographs of the shoulder area of a human patient, FIG. 1a being a satisfactory tomograph, FIG. 1 b being the same tomographrecorded with the aid of a detector system having several defectivechannels and therefore being affected by artifacts, FIG. 1 c being atomograph obtained with the aid of a known correction method from thesame signals as in FIG. 1 b, and FIG. 1 d being a tomograph obtainedwith the aid of the method according to an embodiment of the inventionfrom the same signals as in FIG. 1 b,

FIGS. 2 and 3 show an illustration of the principle of a device forapplying the method according to an embodiment of the invention,

FIG. 4 shows the geometrical relationships fundamental to the methodaccording to an embodiment of the invention, and

FIGS. 5 to 7 show the signals, used for the method, of the twoprojections used, and the point of origin of these signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 2 and 3 show a CT device of the third generation which is suitablefor implementing the method according to an embodiment of the invention.Its measuring arrangement, denoted overall by 1, has an x-ray source,denoted overall by 2, with a radiation aperture 3 placed in front of itand close to the source (FIG. 3), and a detector system 5, constructedas a two-dimensional array of several rows and columns of detectorelements—of which one is denoted in FIG. 2 by D_(k,n)—, with a radiationaperture 6 placed in front of it and close to the detector (FIG. 3). InFIG. 2, for reasons of clarity, only eight rows of detector elementsD_(k,n) are illustrated, but the detector system 5 can have further rowsof detector elements D_(k,n) which is indicated by dots in FIG. 3.

The x-ray source 2 with the radiation aperture 3, on the one hand, andthe detector system 5 with the radiation aperture 6, on the other hand,are arranged opposite each other on a rotary frame 7 in the way to beseen from FIG. 3 and such that a pyramidal x-ray beam, which, during theoperation of the CT device, originates from the x-ray source 2 and iscollimated by the adjustable radiation aperture 3 and whose edge raysare denoted by RS, impinges on the detector system 5. In the process,the radiation aperture 6 is set to correspond to the cross section, setby means of the radiation aperture 3, of the x-ray beam such that onlythat area of the detector system 5 which can be struck directly by thex-ray beam is exposed. In the operating mode illustrated in FIGS. 2 and3, this is eight rows of detector elements D_(k,n), which are denotedactive rows below. The further rows indicated by dots are covered by theradiation aperture 6 and are therefore inactive. Each row of detectorelements D_(k,n) has a number K of detector elements (for exampleK=672), k=1 to K being the so-called channel index. The active rowsL_(n) of detector elements D_(k,n) are denoted by L₁ to L_(N) in FIG. 3,n=1 to N being the row index.

The detector element D_(244,7) is thus the detector element of thechannel k=244 of the 7th detector row (n=7).

The x-ray beam has the cone angle β, plotted in FIGS. 2 and 3, which isthe opening angle of the x-ray beam in a plane containing the systemaxis Z and the focus F. The fan angle φ of the x-ray beam, which is theopening angle of the x-ray beam in a plane lying at right angles to thesystem axis Z and containing the focus F, is plotted in FIG. 2.

The rotary frame 7 can be set rotating about the system axis Z by meansof a drive device 22. The system axis Z runs parallel to the z axis of athree-dimensional rectangular coordinate system illustrated in FIG. 2.

The columns of the detector system 5 likewise run in the direction ofthe z axis, while the rows, whose width b is measured in the directionof the z axis and is 1 mm, for example, run transversely with respect tothe system axis Z and the z axis.

In order to be able to bring an object to be examined, for example apatient P, into the beam path of the x-ray beam, a bearing device 9 isprovided. The bearing device can be displaced parallel to the systemaxis Z, that is to say in the direction of the z axis. Specifically,this can be done in such a way that there is synchronization between therotational movement of the rotary frame 7 and the translational movementof the bearing device. The effect is that the ratio betweentranslational and rotational speed is constant. It is possible to adjustthis ratio by a desired value for the feed h of the bearing device beingselected per rotation of the rotary frame.

It is therefore possible for a volume of an object to be examined, whichis located on the bearing device 9, to be examined in the course ofvolume scanning. Volume scanning may be performed in the form of spiralscanning with the effect that, with simultaneous rotation of themeasuring unit 1 and translation of the bearing device 9, a large numberof projections from various projection directions are recorded via themeasuring unit per revolution of the measuring unit 1. During the spiralscanning, the focus F of the x-ray source moves relative to the bearingdevice 9 on a spiral path denoted by S in FIG. 2.

The measured data read out in parallel from the detector elements ofeach active row of the detector system 5 during the spiral scanning andcorresponding to the individual projections are subjected todigital/analog conversion in a data conditioning unit 10, preferablyarranged on the rotary frame 7, are serialized and transmitted to animage computer 11. For each of the detector elements, the dataconditioning unit 10 includes, in a way not illustrated, an electronicsignal processing unit downstream thereof, that is also denotedelectronic channel unit.

As an alternative to spiral scanning, it is also possible to set thefeed h=0, with the consequence that the focus F moves on a circularpath. This mode of operation is mostly denoted tomograph scanning. Inthe case of tomograph scanning as well, a large number of projectionsfrom various projection directions are recorded per revolution of themeasuring unit 1.

After the measured data, that is to say the projections, have beenpreprocessed in a preprocessing unit 12 of the image computer 11, theresulting data stream passes to a reconstruction unit 13. This unit usesthe measured data to reconstruct CT images of desired layers of theobject to be examined. Specifically in the case of spiral scanning usinga method known per se of spiral interpolation (for example 180LI or360LI interpolation), and using a method likewise known per se in thecase of tomograph scanning.

The CT images are composed of pixels arranged in the form of a matrix.The pixels are assigned to the respective image area, with each pixelbeing assigned a CT number in Hounsfield Units (HU) and with theindividual pixels being displayed in accordance with a CT number/grayvalue scale with a gray value corresponding to their respective CTnumber.

The tomographs reconstructed by the tomograph reconstruction unit 13from the projections are displayed on a display unit 16, for example amonitor, connected to the image computer 11.

Since the CT device in accordance with FIGS. 2 and 3 for implementing amethod according to an embodiment of the invention that is still to bedescribed in detail is designed for restoring the signals of defectivechannels of the detector system 5, the image computer further has asignal restoring device 14 and a buffer 15 assigned to the latter.

The x-ray source 2, for example an x-ray tube, is supplied by agenerator unit 17 with the requisite voltages and currents, for examplethe tube voltage U. In order to be able to set the latter to therespectively requisite values, the generator unit 17 is assigned acontrol unit 18 with a keyboard 19, which permits the values to be setas required.

In addition, the operation and control of the CT device apart from thisis carried out by way of the control unit 18 and the keyboard 19, whichis illustrated by the fact that the control unit 18 is connected to theimage computer 11.

Amongst other things, the number N of the active rows of detectorelements D_(k,n), and therefore the position of the radiation apertures3 and 6, can be set, for which purpose the control unit 18 is connectedto the adjustment units 20 and 21 assigned to the radiation apertures 3and 6. In addition, the rotation time τ can be set, which is the timeneeded by the rotary frame 7 for a complete revolution and which isillustrated by the fact that the drive unit 22 associated with therotary frame 7 is connected to the control unit 18.

In the case of the defect of one or more of the channels of the detectorsystem 5—the defect can be caused by the respective detector elementitself and/or the channel electronic unit downstream of the latter—thesignal of each defective channel is restored on the basis of the methodaccording to an embodiment of the invention.

For this purpose, in a first mode of operation corresponding to a firstvariant of the method according to an embodiment of the invention,during the recording of the respectively current projection the signalof the defective channel that is entirely missing or corrupted inaccordance with the defect respectively present in the currentprojection is restored on the basis of signals, stored in the buffer 15,of the channels, adjacent to the defective channel, of the directlypreceding projection, and on the basis of the signals of thecorresponding channels of the current projection. This is performed inthe signal restoring device 14.

In the first mode of operation, the restoration of the signals of thedefective channel is performed on the basis of the signals of an eightneighborhood (M=8) of channels directly adjacent to the defectivechannel D. For the purpose of simplicity, these channels are denoted inFIG. 5 only by 1 to 8, the defective channel being denoted by D. Here,the channels 4 and 5 are channels that belong to the same detector rowas the defective channel D. The channels 6 to 8 belong to the detectorrow located in the z direction immediately in front of the detector rowcontaining the defective channel D, while the channels 1 to 3 belong tothe detector row arranged in the z direction immediately after thedetector row containing the defective channel D. Thus, if the defectivechannel is located in the detector row L₃, the channels 4 and 5 arelikewise situated in the detector row L₃, the channels 6 to 8 aresituated in the detector row L₂, and the channels 1 to 3 are situated inthe detector row L₄.

During the recording of the current projection, in which in accordancewith FIG. 4 the focus assumes the position F_(a) and the detector theposition 5 _(a), the channels of the eight neighborhood of the defectivechannel D supply the signals sa1 to sa8, as indicated in the left-handpart of FIG. 5. The edge beams for the current projection are denotedoverall by RS_(a) in FIG. 4 for the x-ray beam, and by sa4 and sa5 forthe partial x-ray beam striking the eight neighborhood of the defectivechannel D. The circular measuring field covered by the x-ray beam isdenoted in FIG. 4 by MF.

During the recording of the projection directly preceding the currentprojection, the focus and the detector assumed the positions denoted inFIG. 4 by F_(v) and 5 _(v), the associated edge beams of the x-ray beambeing denoted by RS_(v), and the associated edge beams of the partialx-ray beam being denoted by sv4 and sv5. The output signals, occurringduring the recording of the projection directly preceding the currentprojection, of the eight neighborhood of the defective channel D aredenoted in the right-hand part of FIG. 5 by sv1 to sv8. As mentioned,these signals are stored in the buffer 15.

The restoration of the signal D_(r) of the defective channel for thecurrent projection is performed as follows on the basis of the signalssa1 to sa8 and sv1 to sv8.

The signal restoring device 14 uses the signals of these channels in afirst restoring step to calculate a preliminary correction value v using

$v = {\frac{1}{6} \cdot {\left( {{sa4} + {sa5} + {sv4} + {sv5} + {sf4} + {sf5}} \right).}}$

In order to detect the influence of object structures changing in afashion perpendicular to the image plane (plane of the drawing in FIG.4), the signal restoring device 14 similarly calculates preliminarycorrection values o and u for the detector rows above and below the rowcontaining the defective channel:

$o = {{\frac{1}{6} \cdot {\left( {{sa1} + {sa3} + {sv1} + {sv3} + {sf1} + {sf3}} \right).u}} = {\frac{1}{6} \cdot {\left( {{sa6} + {sa8} + {sv6} + {sv8} + {sf6} + {sf8}} \right).}}}$

The restored signal D_(r) is calculated by the signal restoring device14 from the preliminary correction values v, o and u, preferably usingD _(r) =v−0.5·(o−sa2+u−sa7),or else usingD _(r) =v·0.5·[(sa2/o)+(sa7/u)].

In the way described above for a current projection, the procedureduring the scanning operation for all the recorded projections is suchas to produce a data stream that is corrected with regard to the signalsof the defective channel and which the tomograph reconstruction unit 13accesses in order to reconstruct the tomographs. That is to say, thetomographs are reconstructed by the tomograph reconstruction unit 13 onthe basis of the projections containing the respectively restoredsignals of the defective channel and, as mentioned, are displayed on thedisplay unit 16. The procedure is as described in this case,independently of whether spiral scanning or tomograph scanning isimplemented.

As is clear from FIG. 4, the x-ray source 1 and the detector system 5move jointly on a spiral or circular trajectory about the system axis Z.The system z axis is perpendicular to the plane of the drawing in FIG.4. The signals of the individual projections are detected in each casein an angular position, differing from the directly precedingprojection, of the connecting line of the focus F and the middle of thedetector 5.

The method according to an embodiment of the invention utilizes thisfurther movement of the focus F and the detector system 5 fromprojection to projection. Specifically, this further movement and theuse of signals from the neighborhood of the defective channel of therespective current projection and the respective directly precedingprojection firstly permit the restoration, that is to say approximation,of the signal of the defective channel by way of mean values fromadjacent signals in the way described. This ensures that the beam pathon which the adjacent signals are based deviates on average as little aspossible, owing to the object to be examined, from the beam pathbelonging to the defective channel D.

Each contribution, resulting from the beam path of the x radiation fromthe focus F to the defective channel, to the attenuation of the initialintensity of the x radiation originating from the focus F is thereforeapproximated particularly well by contributions of the signals, used foraveraging, of the channels from the neighborhood of the defectivechannel D. The basis in this case is the selection of those neighboringchannels of a defective channel that are located in the same detectorrow and for which the mean distance of the connecting line from thelocation of the tube focus at the instant of the recording of theprojection to the middle of the channel from the connecting line fromthe location of the tube focus to the defective channel of the currentprojection is smallest.

This results overall in the most accurate approximation possible. Anapproximation of even only approximately similar accuracy would beimpossible without the utilization of the further movement described orwithout the use of signals from the current projection and at least onefurther projection—directly preceding projection in the case of the modeof operation described.

Thus, the existence of uncorrupted signals of the channels of the eightneighborhood of the defective channel is presupposed in the restoringvariant shown.

The buffer 15 can be very small in the first mode of operation, since itis always only the data of the respective preceding projection that mustbe present and are able, after the correction of the data of the currentprojection, to be overwritten thereby, since these data for their partconstitute the data of the preceding projection for the correction ofthe next (current) projection.

In the case of an alternative second mode of operation that can beselected by way of the keyboard 19, a variant of an embodiment of theinvention is applied for which the signal of the defective channel isrestored in a way similar to the first mode of operation but, inaccordance with FIG. 6, by using three temporally adjacent projections.This variant can be applied for restoring signals of defective channelsof the average projection viewed in terms of time. Thus, the signals sf1to sf8 are additionally taken into account then.

Mean values over the corresponding six signals of the respectivedetector row are again to be formed in each case for v, o and u.

The preliminary correction value v is calculated by the signal restoringdevice 14 usingv=⅙·(sa4+sa5+sv4+sv5+sf4+sf5)

It holds for the preliminary correction values o and u that:o=⅙·(sa1+sa3+sv1+sv3+sf1+sf3)u=⅙·(sa6+sa8+sv6+sv8+sf6+sf8).

The restored signal D_(r) is calculated by the signal restoring device14 from the preliminary correction values v, o and u, preferably in away similar to the variant described above, usingD _(r) =v−0.5·(o−sa2+u−sa7)or usingD _(r) =v·0.5·[(sa2/o)+(sa7/u)].

In the case of a third mode of operation, which can be selected by wayof the keyboard 19 as a further alternative, a variant of an embodimentof the invention is applied in which the signal of the defective channelis restored by using the signals of a twenty-four neighborhood (M=24) onthe basis of the respectively current projection and the projectionpreceding the latter.

Consequently, in accordance with FIG. 7 signals sa9 to sa24 and sv9 tosv24 are also taken into account. Assuming that the defective channel issituated in the detector row L₃, signals sa23, sa4, sa5 and sa15 as wellas sv23, sv4, sv5 and sv15 belong to the channels 23, 4, 5 and 15likewise situated in the detector row L₃. Signals sa22, sa6, sa7, sa8and sa16 as well as sv22, sv6, sv7, sv8 and sv16 belong to the channels22, 6, 7, 8 and 16 situated in the detector row L₂; signals sa21, sa20,sa19, sa18 and sa7 as well as sv21, sv20, sv19, sv18 and sv17 belong tothe channels 21, 20, 19, 18 and 17 situated in the detector row L₁. Bycontrast, signals sa24, sa1, sa2, sa3 and sa14 as well as sv24, sv1,sv2, sv3 and sv14 belong to the channels 24, 1, 2, 3 and 14 situated inthe detector row L₄; signals sa9, sa10, sa11, sa12 and sa13 as well assv9, sv10, sv11, sv12 and sv13 belong to the channels 9, 10, 11, 12 and13 situated in the detector row L₅.

The signal restoring device 14 calculates the preliminary correctionvalue v from the signals of these channels usingv=0.125·(sv23+sv4+sv5+sv15+sa23+sa4+sa5+sa15).

In order to detect the influence of object structures changing in afashion perpendicular to the image plane (plane of the drawing in FIG.4), there is now a need for four preliminary correction values o, p andu, w for the detector rows situated in the z direction before and afterthe detector row containing the defective channel:o=0.125·(sv9+sv10+sv12+sv13+sa9+sa10+sa12+sa13)p=0.125·(sv24+sv1+sv3+sv14+sa24+sa1+sa3+sa14)u=0.125·(sv22+sv6+sv8+sv16+sa22+sa6+sa8+sa16)w=0.125·(sv21+sv20+sv18+sv17+sa21+sa20+sa17+sa17).

The signal restoring device 14 calculates the restored signal D_(r) fromthe preliminary correction values v, o, p, u and w preferably using

$D_{r} = {v - {0.5 \cdot \left( {\frac{o + p}{2} - \frac{{sa2} + {sa11}}{2} + \frac{w + u}{2} - \frac{{sa7} + {sa19}}{2}} \right)}}$or using

$D_{r} = {v \cdot 0.5 \cdot \left( {\frac{{sa2} + {sa11}}{o + p} + \frac{{sa7} + {sa9}}{w + u}} \right)}$

The third mode of operation can also be modified in a way similar to thesecond mode of operation to the effect that signals of a projectiondirectly following the current projection are also incorporated into thecorrection in addition to the signals of the projection directlypreceding the current projection.

In the exemplary embodiments described, the influence of signals ofneighboring detector rows is taken into account in each case with aweighting factor of one. However, it is also possible to apply weightingfactors of less than one that take account of the distance, measured inthe z direction, of the respective detector row from the detector rowcontaining the detective channel, the weighting factor being smaller thelarger the distance.

Signals of only one defective channel are restored in the case of theexemplary embodiment described. In a similar way, it is also possible torestore the signals of several defective channels, the precondition forthe applicability of the invention being that each defective channel issurrounded by a neighborhood of intact channels that exhibits the sizerespectively used.

Within the scope of an embodiment of the invention, the detectorelements can be both detector elements that convert the radiation quantaoccurring directly into electrical signals, and detector elements thatconvert the radiation quanta occurring into electrical signalsindirectly with the aid of a scintillator and a photodiode arrangement.

In the case of the exemplary embodiments described, the relativemovement between the measuring unit 1 and the bearing device 9 isproduced in each case by the bearing device 9 being displaced. However,the possibility also exists within the scope of the invention of lettingthe bearing device 9 be fixed and displacing the measuring unit 1instead of this. The possibility also exists within the scope of anembodiment of the invention of producing the requisite relative movementby displacing both the measuring unit 1 and the bearing device 9.

The conical x-ray beam has a rectangular cross section in the case ofthe exemplary embodiment described. However, other cross-sectionalgeometries are also possible within the scope of an embodiment of theinvention.

A CT device of the third generation is used in conjunction with theexemplary embodiments described above, that is to say the x-ray sourceand the detector system are jointly displaced about the system axisduring imaging. However, to the extent that the detector system is amultirow array of detector elements, an embodiment of the invention canalso be used in conjunction with CT devices of the fourth generation, inthe case of which only the x-ray source is displaced about the systemaxis and cooperates with a fixed detector ring.

To the extent that the detector system has a multirow array of detectorelements, the method according to an embodiment of the invention canalso be used in the case of CT devices of the fifth generation, that isto say CT devices in which the x radiation originates not only from onefocus, but from several foci of one or more x-ray sources displacedabout the system axis.

The CT device used in conjunction with the above-described exemplaryembodiments has a detector system with detector elements arranged in themanner of an orthogonal matrix. However, an embodiment of the inventioncan also be used in conjunction with CT devices whose detector systemhas detector elements arranged in another way as a two-dimensionalarray.

The exemplary embodiments described above relate to the medicalapplication of an embodiment of the invention. However, the inventioncan also be applied outside of medicine, for example, in nondestructivematerials testing by means of a CT device.

The exemplary embodiments described are to be understood merely asexemplary, in particular including with regard to the design of theimage computer 11 described. For example, the signal restoring unit 14and the buffer 15 can also be components of a separate computer unit.

It may be stated in summary that an embodiment of the invention makesavailable a correction method with the aid of which it is possible torestore signals of defectively operating or entirely defective channelsof the data recording system of a medical device, in particular a CTdevice: the signal of a detector channel that is operating defectivelyduring a projection or is entirely defective is calculated from thesignals of the channels of the same projection that are adjacent to thechannel, and from the corresponding signals of the projection orprojections preceding this projection in time and/or following it intime. The calculation can be performed, for example, using a signalrestoring device and a buffer device.

It is therefore possible, in particular, to operate a CT device, inparticular in medical diagnostics, despite defective channels with theleast possible loss in quality of the reconstructed tomographs, and/orto continue operation after the occurrence of a defect. The tomographsreconstructed on the basis of projections containing restored signalscan be evaluated very well and are free of disturbing image artifactseven if the signals of individual detector elements are entirelymissing.

Exemplary embodiments being thus described, it will be obvious that thesame may be varied in many ways. Such variations are not to be regardedas a departure from the spirit and scope of the present invention, andall such modifications as would be obvious to one skilled in the art areintended to be included within the scope of the following claims.

1. A method for restoring a signal of a defective channel of a radiationdetector, comprising: recording projections from different projectiondirections in a multiplicity of channels of the radiation detector,wherein the channels each include a detector element with a downstreamchannel electronic unit; and restoring the signal of the defectivechannel by using adjacent signals of an M neighborhood of the sameprojection and from adjacent signals of an M neighborhood of furtherprojections, wherein the signal of the defective channel is restoredfrom signals of the same projection and a projection directly precedingin time.
 2. The method as claimed in claim 1, wherein the signal of thedefective channel is restored by using adjacent signals of at least oneof an eight (M=8) and twenty-four neighborhood (M=24).
 3. The method asclaimed in claim 2, wherein the signal of the defective channel isrestored from signals of the same projection, of the projection directlypreceding time, and of the projection directly following in time.
 4. Themethod as claimed in claim 2, wherein at least one of signal values andpreliminary correction values required for restoring the signal of thedefective channel are provided with a weighting factor.
 5. The method asclaimed in claim 4 for restoring a signal of a defective channel of aradiation detector that has several detector rows with several detectorelements, wherein the weighting factor is selected in accordance with adistance of a respective detector row from the detector row includingthe defective channel.
 6. A medical device comprising: an image computerimplementing the method as claimed in claim
 2. 7. The method as claimedin claim 1, wherein the signal of the defective channel is restored fromsignals of the same projection, of the projection directly precedingtime, and of the projection directly following in time.
 8. The method asclaimed in claim 1, wherein the radiation detector includes severaldetector rows with several detector elements, and wherein the signal ofthe defective channel is restored with aid of a preliminary correctionfactor μ from channels nearest the same detector row of the sameprojection, and from corresponding channels of the same detector row ofone or more temporally adjacent projections, and with aid of twopreliminary correction values o and u of the same and at least onetemporally adjacent projections, o and u being calculated from signalsof the detector rows adjacent to the detector row containing thedefective channel.
 9. The method as claimed in claim 1, wherein at leastone of signal values and preliminary correction values required forrestoring the signal of the defective channel are provided with aweighting factor.
 10. The method as claimed in claim 9 for restoring asignal of a defective channel of a radiation detector that has severaldetector rows with several detector elements, wherein the weightingfactor is selected in accordance with a distance of a respectivedetector row from the detector row including the defective channel. 11.The method as claimed in claim 1, wherein the signal of the defectivechannel is restored by way of a signal restoring device having a buffer.12. A medical device, comprising: an image computer implementing themethod as claimed in claim
 1. 13. The medical device as claimed in claim12, wherein the medical device a CT device.
 14. A method for restoring asignal of a defective channel of a radiation detector, comprising:recording projections from different projection directions in amultiplicity of channels of the radiation detector, wherein the channelseach include a detector element with a downstream channel electronicunit; and restoring the signal of the defective channel by usingadjacent signals of an M neighborhood of the same projection and fromadjacent signals of an M neighborhood of further projections, whereinthe signal of the defective channel is restored from signals of at leastone of several projections preceding in time and of several projectionsfollowing in time.
 15. The method as claimed in claim 14, wherein thesignal of the defective channel is restored by using adjacent signals ofat least one of an eight (M=8) and twenty-four neighborhood (M=24). 16.The method as claimed in claim 14, wherein the radiation detectorincludes several detector rows with several detector elements, andwherein the signal of the defective channel is restored with aid of apreliminary correction factor μ from channels nearest the same detectorrow of the same projection, and from corresponding channels of the samedetector row of one or more temporally adjacent projections, and withaid of two preliminary correction values o and u of the same and atleast one temporally adjacent projections, o and u being calculated fromsignals of the detector rows adjacent to the detector row containing thedefective channel.
 17. The method as claimed in claim 14 for restoring asignal of a defective channel of a radiation detector that has severaldetector rows with several detector elements, wherein the weightingfactor is selected in accordance with a distance of a respectivedetector row from the detector row including the defective channel. 18.The method as claimed in claim 14, wherein the signal of the defectivechannel is restored by way of a signal restoring device having a buffer.